What is claimed is:
1. A spacecraft embedded in a reference frame rotating relative to inertial space comprising:
actuators for maneuvering the spacecraft with respect to the reference frame;
an attitude measurement device that measures the pitch and roll attitude of the spacecraft with respect to the reference frame;
a control device adapted to keep the roll and pitch angles of the spacecraft close to the commanded roll and pitch angles; and
a harmonic torque estimator adapted to read the commanded angular velocity of the spacecraft relative to an inertial frame, read momentum wheel speeds, read known predicted external torques and combines angular velocity, measured wheel speed and known external torque to produce an estimated observable periodic torque.
2. The spacecraft of claim 1 including a rate measurement device wherein the control device estimates both observable and unobservable periodic torque while the onboard rate measurement device is ON.
3. The spacecraft of claim 1 including a momentum storage device providing a non-zero average momentum perpendicular to the orbital plane.
4. The spacecraft of claim 3 including a rate measurement device wherein the control device estimates both observable and unobservable periodic torque while the onboard rate measurement device is ON.
5. The spacecraft of claim 1 wherein momentum stored in momentum wheels is kept close to ideal periodic momentum trajectory.
6. An attitude control system for a spacecraft comprising:
a harmonic torque estimator adapted to estimate a periodic disturbance torque signal and a periodic momentum profile signal based on disturbances observable from the available spacecraft sensors;
an attitude profile generator adapted to provide reference attitude command and a rate command for the spacecraft to follow;
a feedback control device adapted to measure roll and pitch angles to the commanded roll and pitch angles issued by the attitude profile generator and output a torque that is combined with the periodic disturbance torque estimate to form an actuator torque command; and
a momentum management device adapted to use the estimated periodic momentum profile to improve momentum management of the spacecraft.
7. The attitude control system of claim 6 wherein in the step of combining the periodic disturbance torque signal estimate is a feed forward signal issued by the harmonic torque estimator.
8. The attitude control system of claim 6 wherein the periodic disturbance torque signal estimate is fed forward to actuators in the spacecraft dynamic system to improve yaw pointing.
9. The attitude control system of claim 6 wherein the harmonic torque estimator is adapted to measure harmonic torque from body-fixed angular momentum dynamics, wherein the angular momentum is a combination of wheel momentum and spacecraft angular rate.
10. The attitude control system of claim 6 wherein the harmonic torque estimator is adapted to measure observable harmonic torque from body-fixed angular momentum dynamics, wherein the angular momentum is estimated without rate sensor measurements.
11. The attitude control system of claim 6 wherein spacecraft roll and pitch angles are measured from an earth sensor.
12. The attitude control system of claim 6 wherein the momentum management device is further adapted to issue a thruster torque command when a difference between wheel momentum and a predetermined nominal wheel momentum value exceeds a threshold level, the predetermined nominal value being based on the periodic momentum profile provided by the harmonic torque estimator.
13. The attitude control system of claim 6 wherein the harmonic torque estimator further comprises:
a momentum estimator adapted to estimate a body-fixed angular momentum stored in the satellite and generate a momentum error signal;
a harmonic torque Fourier coefficient estimator adapted to output a set of estimated disturbance torque Fourier coefficients for each of the roll and yaw axes developed from the momentum error signal to the momentum profile generator and harmonic torque generator; and
a harmonic torque generator adapted to translate the estimated disturbance torque Fourier coefficients delivered from the harmonic torque Fourier coefficient estimator into the estimated periodic torque.
14. The attitude control system of claim 13 wherein the harmonic torque estimator further comprises a ground prediction module adapted to output a set of predicted Fourier coefficients based on ground models of solar torques to be combined with the set of Fourier coefficients fed to a momentum profile generator and harmonic torque generator.
15. The attitude control system of claim 13 wherein the harmonic torque estimator further comprises a momentum profile generator adapted to integrate the periodic torques to yield a periodic roll and yaw momentum profile.
16. A method of controlling a spacecraft comprising the steps of:
measuring a pitch and roll attitude of the spacecraft;
generating a control torque command designed to keep the roll and pitch angles close to desired roll and pitch angles; and
estimating a periodic disturbance torque
combining the generated control torque command and the estimated periodic torque to produce an actuator torque command.
17. The method of claim 16 wherein inclusion of the periodic torque in the actuator torque improves yaw pointing.
18. The method of claim 16 further comprising the step of using a harmonic torque estimator to produce the estimated periodic torque.
19. The method of claim 16 wherein the step of estimating a periodic torque further comprises the steps of:
reading a commanded angular velocity of the spacecraft relative to an inertial frame;
reading momentum wheel speeds;
reading known predicted external torques; and
combining the angular velocity, wheel momentum and know external torques to produce the estimated periodic observable torque.
20. The method of claim 16 wherein the step of estimating a periodic torque further comprises the steps of:
reading an angular velocity measurement device
reading momentum wheel speeds;
reading known predicted external torques; and
combining the angular velocity, wheel momentum and known external torques to produce the estimated periodic observable and unobservable torques.
The claims below are in addition to those above.
All refrences to claim(s) which appear below refer to the numbering after this setence.
1. A free-flowing non-deliquescent benzalkonium salt having at least one C8-25 alkyl group and obtained by benzylating a tertiary amine consisting of at least 90% by weight of a single homologue, said salt containing from 3 to 10% by weight of water based on the total weight of said salt.
2. A salt according to claim 1, having an equilibrium water content below 10% based on the total weight of said salt at 40 C. in an atmosphere having 65% humidity.
3. A salt according claim 1 having an inorganic salt content of less than 12% by weight based on the total weight of said salt.
4. salt according claim 1 having an inorganic salt content of less than 9% by weight based on the total weight of said salt.
5. A salt according to claim 1 having an inorganic salt content of less than 0.5% by weight based on the total weight of said salt.
6. A salt according to claim 1 wherein said tertiary amine consists of more than 95%, by weight of said amine, of a single homologue.
7. A salt according to claim 6 wherein the purity of the tertiary amine is 96 to 98%.
8. A salt according to claim 6 wherein the amine feedstock is lauryl dimethylamine.
9. A salt according to claim 1 wherein the water content is reduced to a value of from 3.5 to 9% by weight based on the total weight of said salt.
10. A salt according to claim 8 wherein the water content is reduced to a value of from 3.5 to 9% by weight based on the total weight of said salt.
11. A salt according to claim 2 having an inorganic salt content of less than 12% by weight based on the total weight of said salt.
12. A salt according to claim 2 having an inorganic salt content of less than 9% by weight based on the total weight of said salt.
13. A salt according to claim 2 having an inorganic salt content of less than 0.5% by weight based on the total weight of said salt.
14. A salt according to claim 7 wherein the amine feedstock is lauryl dimethylamine.